hmm, looks like a really intresting idea truely...
as it should be dynamically, you have to save it into the db.
i'd suggest to create a new field in the threadtable and save the banned userids there... (like the buddylist is saved..)
the unban feature can be one extra page, or always displayed for the mods when viewing a thread...
i'd do it if i'd have more time, but unfortunately i don't have enough for my todolist..
i hope at least the tips could help